Router for ISP clients: Gigabit ethernet, N450 or better wireless

I'd personally recommend at least 8 MB of flash, at least 16 MB preferred, for long-term compatibility. For me 802.11ac and 802.11s support are mandatory. Some might forgo the 802.11s support.

What's your favorite enthusiast LEDE/OpenWrt device? is likely over the budget, but What's your favourite cheap LEDE/OpenWrt device? may be more on target. Also What's your favorite cheap 5 GHz-capable device

Edit: Yes thanks, that should have been megabytes for flash sizes